The present invention relates to cellular telecommunications, and more particularly to methods and apparatuses that enable equipment in a cellular telecommunications system to perform channel estimation when the pilot signals of a serving cell and those of one or more neighboring cells collide due to the serving and neighboring cells being synchronized with one another.
The forthcoming Evolved-Universal Terrestrial Radio Access Network (E-UTRAN) Long Term Evolution (LTE) technology, as defined by 3GPP TR 36.201, “Evolved Universal Terrestrial Radio Access (E-UTRA); Long Term Evolution (LTE) physical layer; General description” will be able to operate over a very wide span of operating bandwidths (e.g., 1.4 MHz to 20 MHz) and also carrier frequencies. Furthermore E-UTRAN systems will be capable of operating within a large range of distances, from microcells (i.e., cells served by low power base stations that cover a limited area, such as a shopping center or other building accessible to the public) up to macrocells having a range that extends up to 100 km. In order to handle the different radio conditions that may occur in the different applications, multiple access in the downlink (i.e., the communications link from the base station to user equipment—“UE”) is achieved by Orthogonal Frequency Division Multiple Access (OFDMA) technology because it is a radio access technology that can adapt very well to different propagation conditions. In OFDMA, the available data stream is portioned out into a number of narrowband subcarriers that are transmitted in parallel. Because each subcarrier is narrowband it only experiences flat-fading. This makes it very easy to demodulate each subcarrier at the receiver.
Data rates up to and even beyond 100 Mb/s will be supported for the largest bandwidth, and such data rates will be possible by using a Multiple-Input-Multiple Output (MIMO) scheme in the down-link.
Furthermore LTE technology operates in both synchronized and asynchronous networks. In a synchronized network, all of the base stations (e.g., eNodeBs) use the same timing over the air interface, whereas in an asynchronous network, a base station's air interface timing could differ from its neighbor's. The radio channel properties and characteristics of the received signals vary, depending on whether the network is synchronized or not. To take one example, in the case of synchronized networks, the estimated channel in the downlink is typically a multi-channel estimate of all contributing radio base stations. The channel estimate is degraded because the reference signals collide with one another. By contrast, in an asynchronous network the reference signals most-often collide with data rather than with other cells' reference signals, thereby giving a more random behavior that can be treated as noise.
In order to optimize throughput in the system when there is intercell interference, it is important for the UE to have a good channel estimate not only for its own serving cell, but also for any interfering radio channel. The current state of the art for channel estimation uses Wiener filtering in time and frequency. The noise in the model is typically modeled as colored noise. A conventional frequency-domain model for a received signal, Y, is:Yk,l=Hk,luk,l+Ek,l,where Hk,l is the radio channel for symbol l on sub-carrier k between the serving cell and the UE and Ek,l is additive noise.
A problem with the existing solution is that the presence of a strong interfering cell destroys the channel estimation, so that the channel estimation is made in a low signal-to-noise ratio (SNR) environment. It is therefore desirable to provide methods and apparatuses that provide improved performance over conventional techniques.